effect of main 'reservoir' capacitors before or after standby sw in tube rect. amp

[mikepukmel]

Hi All, I think this has been covered, but after a search didn't find an answer. I understand something about charging a big capacitor. (something). The old Fenders mostly had caps before the standby, so that while the rectifier is warming up, it is also charging up the main reservior caps. I dug through a whole bunch of Fender amps through 60's and 70's, and it looks like they did move the caps a couple of times (before to after, then back to before again).

Marshall seemed to move away from rectifier tubes to ss rect earlier, only a few had tube rectifier. The early JTM45 had a tube rectifier and the reservoir caps were after the standby switch. OK so the old days, 60's-80's, the tubes were built more solid than today.

What Im trying to get a sense of, is the stress on, say, a GZ34 rectifier tube when its been warmed up, and the user switches standby 'off' and the tube has to charge up a 32uf cap, and also the remaining caps, and get the tube plates all up to running voltage.

I poked around on some Marshall threads, and didn't see anyone complain that their GZ34 were getting toasted, but not many JTM45 builders compared to more common ss rect marshalls, it seems.

[galaxiex]

I'm certainly no expert,
but isn't the larger concern the voltage stress on the caps that are pre standby?
Since the unloaded circuit voltages are higher?

[Enzo]

We can start with an answer we expect and then rationalize backwards from there, but really, Marshall, Fender, others, they all do it different ways that ALL WORK. There isn't a right and wrong. Someone will always talk about tube life or cap life or whatever, but frankly, I replace way more caps that are 40 years old because they are old, not because they had this or that sort of standby switch circuit.

And after 40 years, are we really going to make a case that one way might have given us another 6 months?

[tpaairman]

Here's something else to consider. Amps are the only tube devices that have standby switches. I fix antique radios as a hobby, and not one single model of tube radio ever had a standby switch, and no more of a failure rate among radios or TV's than amps due to to the lack of such a switch.

[the fatch]

Charging the reservoir after a rectifier tube is up to working temperature is known as 'hot switching'. The large inrush of current to charge the cap can cause catastrophic arcing in the tube, and so was usually forbidden by tube manufacturers. This information plus much more on the pros and cons of standby switches at http://www.valvewizard.co.uk/standby.html

[R.G.]

One thing I keep getting profoundly happy about is that today, things that were simply impossible a few decades ago are simple and easy to do now.

We no longer have to choose how much to kill the rectifiers and/or caps by hardwiring them one way or another. The current surge issues are simple enough to resolve. There is one circuit to do just this at geofex.com: http://www.geofex.com/Article_Folder...nt%20Clamp.pdf It uses a power MOSFET to watch the currents into the caps and simply flat line them at some value you decide is safe for the rectifier and caps. With a little more circuit gymnastics (ie. an opto isolator) you can soft switch the current clamp off to implement your standby, and take the voltage and current stress off your standby switch.

As long as the current limit is set higher than the pulse currents into the cap at full power out, the circuit has no impact on normal operation at all. All it does is soften the power up surges. Not bad for two transistors, two resistors, and three diodes.

[R.G.]

Here's something else to consider. Amps are the only tube devices that have standby switches. I fix antique radios as a hobby, and not one single model of tube radio ever had a standby switch, and no more of a failure rate among radios or TV's than amps due to to the lack of such a switch.

Forgot to answer this one. The difference is that tube radios were deliberately designed to last a long time, so their designers were more sensitive to the issues of burning out tubes. Guitar amps were historically designed to get the biggest power for the dollar, so certain refinements - like worrying about tube life - probably took a lower importance.

Well, and tube radios didn't usually shovel out 50-100Wrms to the speaker.

[Leo_Gnardo]

Here's something else to consider. Amps are the only tube devices that have standby switches. I fix antique radios as a hobby, and not one single model of tube radio ever had a standby switch, and no more of a failure rate among radios or TV's than amps due to to the lack of such a switch.

We kicked this one around a couple years ago. And I'm sure the same at The Amp Garage. Bottom line was, if you have a multi thousand watt radio transmitter, the kind with football size, x-pensive transmitter tubes, sure then you definitely want them warmed up before applying high voltage. Standby on guitar amps apparently is a convenience to those who want to mute their amps whilst on break at gigs. Besides that, standbys are a sop to those who think they need 'em, and that's about all. They sure do put a hurtin' on rectifier tubes, and amps with main filter caps that can't take the extra high voltage without a load. I used to think some of my customers were nuts for switching both power and standby on when starting up their Fenders. Now, at least for the amps that have slow warmup rectifiers, 5AR4/GZ34 and similar, it's really no problem.

The controversy rages on ...

[mikepukmel]

I'm certainly no expert,
but isn't the larger concern the voltage stress on the caps that are pre standby?
Since the unloaded circuit voltages are higher?

Thanks Galax, yes also found that to be a problem. The first wirig of this amp, many months ago, I had 5 16uf 475v caps, as per the ancient schematic. When I started the amp up and measured around voltages, I found just that: with standby ON (not amplifying), the voltage across the two reservoir caps, crept up closer to 480v range maybe 490v. Not knowing much, I was worried about hitting these caps over rated v for 30 seconds or so EVERY time I turned on the amp, so I replaced them with 2 x 22uf 500v caps. That was around the time I started reading about max capacitance, and what it meant to the rectifier tube. The main reason for asking here, is due to a new amp build, and hoping to get a little more life out of the thing.

[mikepukmel]

We can start with an answer we expect and then rationalize backwards from there, but really, Marshall, Fender, others, they all do it different ways that ALL WORK. There isn't a right and wrong. Someone will always talk about tube life or cap life or whatever, but frankly, I replace way more caps that are 40 years old because they are old, not because they had this or that sort of standby switch circuit.

And after 40 years, are we really going to make a case that one way might have given us another 6 months?

Thanks Enzo. Im more worried that, over the years, you guys have replaced many rectifier tubes, curious how many of the amps were 'reservoir cap before the standby' vs 'reservoir cap after standby'. If its only 6 months longer, no biggie, but if it is likely to kill these new manufactured tubes, and take something else out as well, . . .

[mikepukmel]

Charging the reservoir after a rectifier tube is up to working temperature is known as 'hot switching'. The large inrush of current to charge the cap can cause catastrophic arcing in the tube, and so was usually forbidden by tube manufacturers. This information plus much more on the pros and cons of standby switches at http://www.valvewizard.co.uk/standby.html

Thanks for the link Fatch, reading. Didn't know Merlin had an article.

[mikepukmel]

One thing I keep getting profoundly happy about is that today, things that were simply impossible a few decades ago are simple and easy to do now.

We no longer have to choose how much to kill the rectifiers and/or caps by hardwiring them one way or another. The current surge issues are simple enough to resolve. There is one circuit to do just this at geofex.com: http://www.geofex.com/Article_Folder...nt%20Clamp.pdf It uses a power MOSFET to watch the currents into the caps and simply flat line them at some value you decide is safe for the rectifier and caps. With a little more circuit gymnastics (ie. an opto isolator) you can soft switch the current clamp off to implement your standby, and take the voltage and current stress off your standby switch.

As long as the current limit is set higher than the pulse currents into the cap at full power out, the circuit has no impact on normal operation at all. All it does is soften the power up surges. Not bad for two transistors, two resistors, and three diodes.

How cool is that, thanks RG! Looks like a simple enough ckt and components easy to find.

[mikepukmel]

Forgot to answer this one. The difference is that tube radios were deliberately designed to last a long time, so their designers were more sensitive to the issues of burning out tubes. Guitar amps were historically designed to get the biggest power for the dollar, so certain refinements - like worrying about tube life - probably took a lower importance.

Well, and tube radios didn't usually shovel out 50-100Wrms to the speaker.

With as little as I do know, that was my sorta, kinda, sorta hunch as well. Guitar amps, the "real" ones designed for playing gigs, had such different design requirements!

[mikepukmel]

We kicked this one around a couple years ago. And I'm sure the same at The Amp Garage. Bottom line was, if you have a multi thousand watt radio transmitter, the kind with football size, x-pensive transmitter tubes, sure then you definitely want them warmed up before applying high voltage. Standby on guitar amps apparently is a convenience to those who want to mute their amps whilst on break at gigs. Besides that, standbys are a sop to those who think they need 'em, and that's about all. They sure do put a hurtin' on rectifier tubes, and amps with main filter caps that can't take the extra high voltage without a load. I used to think some of my customers were nuts for switching both power and standby on when starting up their Fenders. Now, at least for the amps that have slow warmup rectifiers, 5AR4/GZ34 and similar, it's really no problem.

The controversy rages on ...

Thanks, Leo, back in the day, I was lucky to get to look over the shoulder of the engineer who maintained the radio transmitters at the radio station in Poughkeepsie. I will never forget that feeling, the fuzz on the back of my neck on edge, when he took the guard panel off one of the big pieces of equipment and showed me the HUGE vacuum tubes in there, and that loud 60hz hum all over the room.

I got the feeling by looking around at used amps for sale, that the bulk of Marshall amps in existence today, the fewest ones were the first JTM45's. Later amps, they moved to ss rectifiers, like the 1986 and such, many, many more of these around for sale. But Fender, except for the bigger amps, like the Twin, many had tube rectifiers, and they were mostly all sb switch after first cap type

Leo, if its a slow warmup tube rect or not, and you get the tube rectifier all warmed up and happy, then flick the standby off, won't that cause the waterfall out of the tube into that empty 32uf cap?

I get to a point where I think I have a qualitative understanding, but no where quantitative enough to figure out real numbers, then maybe not so much.

I do remember those first tube amps we had back in the day, really beating the hell out of them. Poor things. Wonder any of them survived.

Do any of you guys who do and have done many, many amp repairs, make mods like moving the standby switch after the filter cap, if you found a problem?

Of course its blasphemy to suggest that to someone who just paid $20,000 USD for a vintage unmodified Marshall kitchen amp.

***

If the tube rectifier does pop, do they usually go short or open? Or no way to tell?

[mikepukmel]

One thing I keep getting profoundly happy about is that today, things that were simply impossible a few decades ago are simple and easy to do now.

We no longer have to choose how much to kill the rectifiers and/or caps by hardwiring them one way or another. The current surge issues are simple enough to resolve. There is one circuit to do just this at geofex.com: http://www.geofex.com/Article_Folder...nt%20Clamp.pdf It uses a power MOSFET to watch the currents into the caps and simply flat line them at some value you decide is safe for the rectifier and caps. With a little more circuit gymnastics (ie. an opto isolator) you can soft switch the current clamp off to implement your standby, and take the voltage and current stress off your standby switch.

As long as the current limit is set higher than the pulse currents into the cap at full power out, the circuit has no impact on normal operation at all. All it does is soften the power up surges. Not bad for two transistors, two resistors, and three diodes.

Hi R.G., have you ever built one of these? Is it safe to put inside a Fender style chassis, gets pretty hot in there. Maybe wire the mosfet outside the chassis someplace, with suitable safety shield around the HV wiring, e.g. cap can?